Panzerschwein,
Adjustable powder measures did exist, although I don't know how far back. This one, from Gary Hendershott's
Comancheria Collection, is on the strap for a Comanche shot pouch:
I saw a very good copy of an original in a post on another muzzleloading forum a while back, but I was unable to locate it this evening. This replica, I believe, is still in production, although I don't remember who was selling it. Sorry... I don't take notes on everything I see and read, and now and then I wish I did!
I would agree that the owner of a trade gun would have most likely had a simple, non-adjustable measure. Trade
rifles were often issued with tubular tin powder measures. these were described in an article in the
Museum of the Fur Trade Quarterly, Vol. 12, No. 4 (1976).
Hot Dip Tin makes an excellent replica of one of these, citing that article as a reference. They only offer these in a 55-grain capacity, which is right at two drams. I don't know if you could talk him into making a larger one or not. Wouldn't hurt to ask, once you are sure of the charge you want.
I have not seen a reference indicating these tin powder measures were issued with smoothbored trade guns. However, there are a lot of native-made powder chargers in museum collections. For starters, check out the searchable collections database for the
National Museum of the American Indian. Using "powder measure" as a search term, it pulled up 59 examples from the collection for me.
Another thing to consider as a non-adjustable powder measure would be a brass cartridge case of an appropriate early caliber of rifle. There was a considerable period of overlap in the use of muzzleloaders (including flintlocks) and breechloaders on the western frontier. I have seen several examples of spent .50-70 cases paired up with a powder horn, for use as a powder measure. The brass case was frequently cut off at a slant, to make a sort of pouring spout.
That doesn't help you much if you want a period-correct adjustable measure, though. I'll see if I can find any more about the reproduction mentioned above, but I think your best bet is to work up a load and then either make or order a traditional non-adjustable charger in the size you need, to keep with your gun.
Good luck!
Notchy Bob
ADDENDUM (Edit): The
Log Cabin Shop in Lodi, Ohio has the 19th-century styled adjustable powder measures that I was thinking of. Click that link,and look for the "Custom Brass Powder Measure" and the "Custom Iron Powder Measure." The brass and iron ones look about the same, except for the material, and they are the same price.